1. Field of the Invention
The present invention relates to a method and apparatus for positioning a head on the basis of a premeasured amount of displacement.
2. Description of the Related Art
Disks used in magnetic disk apparatuses include a floppy disk and a hard disk. The hard disk has a thermal offtrack problem. The floppy disk has a humidity offtrack problem and a problem of eccentricity at the time of disk change, as well as the thermal offtrack problem. Therefore, it is technically very important to position a head to a destination track at high speed with high precision. For this purpose, servo data is written in the disk. The servo data are classified into dedicated servo data, sector servo data, and index servo data in accordance with schemes for writing the servo data.
In a conventional servo disk apparatus, servo data is read out from a disk and tracking control is performed in accordance with an amount of displacement upon detection of displacement derived from the servo data. That is, closed loop positioning control is performed. For example, in a case where an intermittent servo scheme such as a sector or index servo scheme is employed, the head position is adjusted in accordance with an amount of displacement derived from servo data of the current head position while the head is moved from the current head position to a position at which the next servo data is written. In a continuous servo scheme such as the dedicated servo scheme, servo data can be continuously obtained. This scheme has higher precision than the intermittent servo scheme, but is in fact the closed loop positioning control. The above problem will be described by exemplifying the sector servo scheme with reference to FIG. 1. When a displacement d1 is detected at a given sector, the head position is controlled to compensate for the displacement d1. However, when a disk-dependent displacement d2 is present, this displacement cannot be eliminated.
In the conventional control scheme, tracking control is performed in accordance with the detected amount of displacement. Therefore, if anisotropic variations such as eccentricity are present, the tracking operation is markedly delayed for a medium such as a floppy disk, or accurate tracking cannot be performed, resulting in inconvenience.